1. Field of the Invention
The present invention relates generally to couplings used to connect pipe segments to form a string, and more particularly relates to couplings adapted to connect sucker rods together to form a sucker rod string for use in a producing well.
2. Description of the Related Art
Sucker rod couplings which connect individual sucker rods to form a string are a key component in the successful performance of a sucker rod string. By minimizing the wear breakage and corrosion of couplings, the life of the sucker rod string is increased, which can substantially decrease service and repair costs. However, sucker rod couplings can be the weakest link in the sucker rod string, limiting the amount of stress that the sucker rod string can absorb without failure.
The general industry practice has been to design sucker rod strings so that stress range requirements of the particular well application are within the allowable limits of the sucker rod body. This practice, however, erroneously assumes that the sucker rod couplings will provide the same or greater service life. This assumption is not valid for certain combinations of rods and couplings and results in derating the allowable coupling loads to give satisfactory coupling stress levels and corresponding service life.
Derating refers to the determination of the ultimate tensile strength of a particular coupling and sucker rod combination by multiplying the ultimate tensile strength of the sucker rod by a suitable derating factor. The derating factor normally has a value less than 1 and varies depending on the particular combination of coupling size and sucker rod size, and material strength.
There are two common situations involving derating. The first case involves the use of American Petroleum Institute (hereinafter "API") slimhole couplings with standard API or high strength grades of sucker rod. Although the ultimate tensile strength of the coupling and rod may be similar, the reduced wall thickness of slimhole couplings reduces the effective stress area thus creating an increase in stress in the coupling for any given load. For example, a typical derating factor for a 1 inch (approximately 0.025 m) diameter API slimhole coupling, when used with an API grade D sucker rod, is approximately 0.89. Thus, the combination of a 1 inch (approximately 0.025 m) diameter API slimhole coupling with an API grade D rod can typically withstand only about 89% of the stress that the API grade D rod could sustain alone.
The second scenario involves the use of a full-size API coupling with a high strength sucker rod which typically has an ultimate tensile strength in excess of 115,000 pounds per square inch (hereinafter "psi"). Here the ultimate tensile strength of the high strength sucker rod exceeds the ultimate tensile strength of the full-size API coupling. For example, a derating factor for a 7/8 inch nominal size (approximately 0.019 m) diameter full-size API coupling in combination with a high strength sucker rod is approximately 0.85. Thus, the combination of a 7/8 inch (approximately 0.019 m) full-size API coupling with a high strength sucker rod can withstand only about 85% of the stress that the high strength sucker rod could withstand alone.
Corrosive well environments further complicate the problem. Sour gas wells may cause sulfide cracking in coupling cores, or ordinary corrosion may cause the cores to fail. A thin nickel based metallic alloy is typically applied to the outer surface of a sucker rod coupling to protect the coupling from corrosion and sulfide cracking. However, such coatings are themselves susceptible to stress cracking when applied to coupling core materials that have a higher hardness and ultimate tensile strength, or when applied to a coupling core material that is to be heat treated. The surface coating tends to become brittle during the subsequent heat treatment.
Conventional fabrication techniques have failed to produce a sucker rod coupling of sufficient fatigue strength, ultimate tensile strength, and resistance to corrosion and surface cracking to allow operators to fully utilize the capability of existing sucker rods.